The Broad View of Holocene Climate from the Swedish ScandesVolume 16, Number 50: 11 December 2013

In the introduction to his recent contribution to the cause of deciphering the history of climate change over the entire course of the Holocene, Kullman (2013) writes that "in central and northern Scandinavia, our understanding of the Holocene tree line history has been progressively refined over the past 50 years or so by the use of radiocarbon-dated mega-fossil tree remains, preserved above the modern tree line elevation in peat and lakes and under glacier ice," citing the studies of G. Lundqvist (1959), J. Lundqvist (1969), Karlen (1976), Eronen (1979), Aas and Faarlund (1988, 1999), Nesje et al. (1991), Moe and Odland (1992), Kullman (1995, 2000), Selsing (1998), Kullman and Kjallgren (2000, 2006), Helama et al. (2004), Paus (2010) and Oberg and Kullman (2011a, b). And in further expounding on the subject, the Swedish scientist reassuringly states that "fundamental aspects of glacial, Lateglacial and early Holocene plant cover and landscapes in northern Scandinavia, as inferred by this approach (Kullman 2006, 2008), are consistent with independent fossil, genetic and palaeo-geographical data," citing Paus (2010), Paus et al. (2011), Carcaillet et al. (2012) and Parducci et al. (2012). In fact, he contends that mega-fossil analysis is the only methodology that can "accurately document the existence of a certain tree species at a certain spot and at a certain point of time in the past."

Consequently, Kullman goes on to develop what he refers to as "a synthesis of all previously published - and some new - mega-fossils representing principal tree species in the south and central Swedish Scandes." More specifically, the Swedish scientist's study includes a sample of 455 radiocarbon-dated mega-fossils (258 Pinus, 172 Betula, 25 Alnus) that originated from the present-day tree line ecotone and above, thirteen of which had not previously been published. And the resulting history he developed indicates that "summer temperatures during the early Holocene thermal optimum may have been 2.3°C higher than present," which difference, in Kullman's words, "corresponds to a general cooling trend of 0.24°C/century, which matches the Milankovitch model of orbitally-driven climate forcing (cf. Berger, 1988; Esper et al., 2012) and indicates that this mechanism has operated as the ultimate driver of climate change throughout the Holocene."

Focusing on the last two millennia, Kullman states that "the pine tree line was about 100 m higher than today (i.e., early 21st century) c. 1940 and 1300-930 cal. years BP," while noting that "the same applies to birch by c. 1700 and 1300 cal years BP," which clusters "represent the Medieval and Roman times." And he writes that "mega-fossil and tree-ring studies in the northernmost part of the Scandes and adjacent regions display broadly the same features," citing the studies of Karlen and Kuylenstierna (1996), Karlen (1998), Hiller et al. (2001), Shivatov (2003), Kremenetski et al. (2004), Moberg et al. (2005) and Esper et al. (2012), after which he says that "these temperature anomalies were succeeded by a distinct tree line/temperature dip, broadly corresponding to the Little Ice Age."

In commenting further on these findings, Kullman says that "the emergence during the past two millennia of at least two short-term tree line and thermal excursions to higher than present levels (i.e. early 21st century) indicates that the current performance of the ecological and climatic systems is well within the envelope of the natural variability of the late Holocene (cf. Karlen, 2008; Akasofu, 2010; Curry and Webster, 2011; Humlum et al., 2011; Kobashi et al., 2011; Ljungqvist et al., 2012)." And to make this point perfectly clear, Kullman repeats that "the pine tree line (and summer temperature) was consistently higher than present ... during the Roman and Medieval periods, c. 1900 and 1000 cal years BP."

Thus, as we have long contended, and on the basis of real-world data (properly analyzed), Kullman's analysis of tree-line data, along with the results of the many other studies he cites, jointly provide yet another strong amalgamation of evidence that supports the view that there is nothing unusual, unnatural or unprecedented about earth's current level of warmth.